Apparatus for the rapid electrometric determination of ion activity

ABSTRACT

An apparatus for the rapid electrometric determination of ionic activities, especially the pH-value, p(Me)-value or p(L)-value of substances, by means of an ion sensitive or ion specific sensor of low or extremely high internal resistance. There is also provided a preferably transistorized amplifier circuit arrangement and an indicator device for the indication of the measurement values in analogue or digital form. According to the invention there is provided as the sensor an electrode measuring chain possessing measuring- and reference electrode means, such electrode-measuring chain exhibiting a clearly defined point of intersection of the isotherms, a constant electrical chain null point, an internal buffer of high buffer capacity and stable slope. A storage vessel is also provided for the measuring chain, and this storage vessel contains an activation solution to maintain said measuring chain under the influence thereof. Fixedly balanced resistor means for said amplifier circuit arrangement control the asymmetrical potential, the electrical null point of the apparatus, the temperature factor of the measuring chain and the amplification factor.

[451 Aug. 14, 1973 APPARATUS FOR THE RAPID ELECTROMETRIC DETERMINATIONOF ION ACTIVITY Wlly Walter Wlrz, Austrasse 9, Meilen, SwitzerlandFiled: Apr. 26, 1971 Appl. No.: 137,431

Inventor:

[30] Foreign Application Priority Data Apr. 24, 1970 Switzerland 6235/70Apr. 19,1971 Germany P 21 18 876.1

US. Cl. 324/30 C, 204/195 G G0ln 27/42, BOlk 3/00 Field of Search324/29, 30 R, 30 B, 324/30 A, 30 C; 204/1 R, l T, 195 G Pettit, F. 5.;Thermodynamic and Elect. ;J. of Phy. Chem.; V01. 68; No. 1;.lan. 1964;pp. 9-13; copy 9-13.

Primary Examiner-Alfred E. Smith Assistant Examiner-Ernest F. KarlsenAttorney-Karl W. Flocks ABSTRACT An apparatus for the rapidelectrometric determination of ionic activities, especially thepl-l-value, p(Me)- value or p(L)-value of substances, by means of an ionsensitive or ion specific sensor of low or extremely high internalresistance. There is also provided a preferably transistorized amplifiercircuit arrangement and an indicator device for the indication of themeasurement values in analogue or digital form. According to theinvention thereis provided as the sensor an electrode measuring chainpossessing measuringand reference electrode means, suchelectrode-measuring chain exhibiting a clearly defined point ofintersection of the isotherms, a constant electrical chain null point,an internal buffer of high buffer capacity and stable slope. A storagevessel is also provided for the measuring chain, and this storage vesselcontains an activation solution to maintain said measuring chain underthe influence thereof. Fixedly balanced resistor means for saidamplifier circuit arrangement control the asymmetrical potential, theelectrical null point of the apparatus, the temperature factor of themeasuring chain and the amplification factor.

16 Claims, 2 Drawing Figures l ""7" I 4* 2| M :2 u 7 IO 5 Q 9 2| -E'T-"l 6 I I ::;::::i'::::

S l I PMENTEUAIIB 14 M6 SBEEI 2 (IF 2 APPARATUS FOR THE RAPIDELECTROMETRIC DETERMINATION OF ION ACTIVITY BACKGROUND OF THE INVENTIONI of the measurement result in analogue or digital form.

As is known the pH-value is a measure of the acid,

neutral or alkali reaction of a substance in aqueous solution and isdefined by pH -log a,, and the p(Me)-value and p(L)-value are measuresfor the activity of specific Me and L-ions (cations and anions).

in order to obtain exact or reasonably exact measure ment results duringthe electrometric determination of ion activity with heretofore knowndirect indicating devices it is required to calibrate the device beforeeacn measurement with the aid of standard solutions (calibrationbuffers) and in the case'of prolonged series of measurements topost-calibrate the device. It is required for such measuring devices,for instance a pH- meter that the asymmetric potential of the measuringchain and the deviation of the actual measuring chain slope from thetheoretical must be capable of correction. To this end, the electricalnull point of the device and a counter-potential derived for instancefrom the amplifier as well as the amplification factor of the measuringdevice are generally designed to be variable. Thus, the measuringdevices are generally equipped with adjustable or variable voltagedividerswhich can be manually controlled by rotary knobs. Oftentimeshowever the adjustment of the electrical null point or zero setting ofthe device and the taking into account of temperature influences occursautomatically, for instance by means of an electronic control, which inthe base 'of temperature corrections contains a temperature-dependcntresistor. Thus the prior art measuring devices always possess anappropriate number of operating elements which, during calibration andpostcalibration with the aid of standardized solutions, must beoperatedin a predetermined manner. Oftentimes there is also provided aselector switch for the adjustment of the different functions. By virtueof these measures whichare considered to be absolutely necessary themeasurement of, for instance, pH-values, is both cumbersome andtime-consuming. Attempts have been made to simplify the use of measuringdevices, for instance with the aid of a programmed perforated disc foradjusting the variable voltage divider and/or by diagram orgraph-reading scales beneath the measuring device indicator for takinginto account the different measurement temperatures. Nonetheless anappreciable progress could not be realized and a decisive simplificationin the use of the device, for instance a pH meter, is also notobtainable as long as the previously discussed requirements areconsidered absolutely necessary.

Since the use of such devices is complicated and cumbersome as apractical matter the carrying out of measurements regarding ion activitycan only be effectively performed by experts or persons who haveundergone prolonged training. If the device is used by unskilled orunschooled individuals generally erroneous measurement results anderroneous evaluation results arise owing to errors resulting fromimproper adjustments and read-out, inexact standardized solutions anderroneous or inaccurate adjustments of the operating elements.Consequently, measurements concerning ionic activity are generally onlyperformed in research and industry, notwithstanding the fact that itwould be desirable if in particular pH-measurements were carried outmore extensively in a wider field, especially in different businesses,governmental agencies and schools. Thus for instance it would beadvantageous and useful to carry out, for example pit-measurements ofthe water of small and large installations as rapid control of forinstance the bathing water in swimming pools and public baths, indoorpools, spring water, underground water, drinking water, water fromlakes, and waste water of all types, for all types of laboratories,during washing and flushing processes, for animal husbandry, medicineand biology, for ground samples or probes in agriculture andhorticulture, for the control of foodstuffs and food products, and soforth.

SUMMARY OF THE INVENTION Accordingly, it will beunderstood from theforegoing that the prior art is still in need of apparatus for the rapidelectrometric determination of ion activity in a manner free from theaforementioned disadvantages present in the state-of-the-art equipment.Therefore, a primary object of this invention is to provide novelapparatus of the aforementioned character which effectively and reliablyfulfills the existing need in the art and overcomes or at leastconsiderably minimizes the drawbacks existant in the prior artconstructions.

Another and more specific object of the instant invention relates to theprovision of a new and improved type of apparatus for the rapid orhigh-speed electrometric determination of ion activity, such apparatusbeing relatively simple and economical to manufacture, especially easilyusable even by inexperienced personnel, and further capable ofdelivering in practice sufficiently accurate measurement results, andwherein as a practical matter erroneous measurements and erroneousevaluations of the measurement results do not occur, and further whereinthe inventive apparatus possesses as usual an ion selective sensorcoupled with an amplification circuit arrangement after which there isconnected a measuring device.

Now these and still further objects of the invention will become morereadily apparent as the description proceeds, and in order to implementthe above objects the invention contemplates that the sensor embodies aconventional or known electrode measuring chain possessing ameasuringand reference electrode and having a clear or definedisothermic intersection point, a constant electrical chain null point,an internal buffer of high buffer capacity and stable slope.Furthermore, there is provided for the measuring chain a storage vesscicontaining an activation or sensitization solution in order to maintainthe measuring chain under the influence of the activation solution.Additionally, fixedly balanced or matched resistors are arranged in theelectrical circuit of the amplifier for the asymmetrical potcntial andthe electrical null point of the device, for the temperature factor ofthe measuring chain and the amplification factor or gain. Electrodemeasuring chains of the required type at commercially available on themarket. Owing to the continuous activation brought about by theactivation solution located externally of the reference electrode it isensured that the asymmetrical potential and the slope of the measuringchain remain constant. After a preliminary or initial calibration,undertaken for instance by the manufacturer, all further measurementstake place without the need to perform switchingand calibrationoperations, and it is not necessary as was heretofore the case toperform at the device correction adjustments while using calibrationbuffers. The measurement value is read-off directly and interpreted inaccordance with tables for the relevant range of use.

It is preferable to use an electrode measuring chain, the referenceelectrode wire of which is immersed in a conductive electrolytecontaining stabilization additives, and in the storage vessel there isfilled an activa tion solution, the chemical composition of whichcorresponds to or is the same as that of the conductive electrolyte. Thepurpose of the stabilization additives is to prolong the longevity ofthe electrode measurement chain, to render the measurement or measuringchain insensitive to different contaminants, to prevent clogging of themembrane or diaphragm of the reference electrode owing to calcificationor fungus growth and therefore to ensure that the asymmetrical potentialand slope of the measurement chain are constant.

The stabilization additive of the activation solution, also used asconductive electrolyte and potential determining agent for the referenceelectrode, preferably consists of at least two of the followingsubstances: alkali hydrogen phthalate, alkali tetraborate, alkaliphosphate, alkali hydrogen phosphate, thymol, alkylester of hydroxybenzoic acid, alkali benzoate, citric acid, acetic acid, monochloroacetic acid, alkali salt of the acetic acids and the monochloro aceticacids. These additives are free of halogen ions supplied by thepotassium chloride in constant concentration of the base electrolyte,and determine the potential of the reference or base system (half cells)as will be further explained hereinafter. Further, the additives arefree of heavy metal ions and do not form together with the half cells ofthe measuring chain any disturbing complex ions.

Simply formulated and effective activation solutions are composed asfollows:

Base electrolyte, potassium chloride about 22.0

Sodium chloroacetate about 2.8 k

Monochloro acetic acid about 2.0

Base electrolyte, potassium chloride about 23.0

Sodium acetate about 0.8

Acetic acid about 0.6

Base electrolyte, potassiumor sodium chloride 23.5

Potassium dihydrogen phosphate KHJO, 0.34

Disodium hydrogen phosphate NaJIPO 0.36

Thyrnol 0.01

All three compositions further contain traces of silver chloride forsaturation, for Ag/AgCl/KCl-reference electrode parts. TheKCl-concentration of the activation solution used within and externallyof the reference electrode or single rod-measuring chain, together withthe stabilization additives, determine the the stable chain null pointof the electrode measuring chain, and there is obtained the precise,one-time calibration, which is also stablethroughout a certaintemperature range, as will be still further more fully explainedhereinafter.

The amplifier circuit arrangement can consist of a known bridge circuitof two amplifiers, each of which is provided with a respectivefield-effect transistor as the input amplifier element. At the amplifieroutputs there is connected an output element for the analogue or digitalprocessing of the output signals and at the output element there isconnected an indicator device. The gate electrode of the field-effecttransistor of one of the amplifiers of the bridge circuit can beconnected with a fixed-balanced voltage divider for the asymmetricalpotential and the electrical null point or zero setting of the deviceand its amplifier output can be connected with the output element bymeans of a fixedly adjusted, variable resistor for the temperaturefactor of the measuring chain and the amplification factor.

In order to be able to perform measurements with grounded andnon-grounded sample substances the signal input of the amplifier of thebridge circuit containing-the fixedly balanced resistors, withnetworkand housing grounding of the electronic circuit, can be groundedby a capacitor. In order to dampen the switch-in pulse and to render thecircuit arrangement symmetrical it is advantageous to connect the gateelectrode of the fieldeffect transistor of the one amplifier of thebridge circuit with the fixedly balanced voltage divider by means of theresistor of an RC-element, and at the other amplifier a capacitor isconnected with the gate electrode of its field-effect transistor, thiscapacitor forming a second RC-element together with the resistance ofthe electrode of the measuring chain which is coupled with the gateelectrode.

In accordance with a particularly simple to operate apparatus theamplifier circuit arrangement with the output element is accommodated ina closed housing in which there is also arranged the storage vessel forthe activation solution and the electrode measurement chain. The storagevessel contains a switch mechanism which switches in the amplifiercircuit arrangement during removal of the electrode measuring ormeasurement chain out of the storage vessel and which switches out suchamplifier circuit arrangement when introducing the electrode measuringchain.

In accordance with a particularly simple constructional manifestation ofswitching mechanism there can be provided at the floor or bottom of thestorage vessel a silicone rubber profile member which seals in watertight fashion a hollow compartment in which there is arranged a switchwhich can be actuated by exerting a pressure upon the diaphragm ormembrane.

The invention will be explained in its particular and specific aspectsin conjunction with an example of carrying out pH-measurements and inconjunction with an embodiment of inventive apparatus of extremelysimple constructional design.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood and objects other than those set forth above, will becomeapparent when consideration is given to the following detaileddescription thereof. Such description makes reference to the annexeddrawings wherein:

FIG. 1 is a schematic sectional view of a preferred embodiment ofapparatus designed according to the inventive teachings and serving tomeasure pH-values; and

FIG. 2 is a circuit diagram of the amplifier circuit arrangement used inthe apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now thedrawings, and as best seen by referring to FIG. I, the apparatusillustrated therein and serving to measure pH-values comprises acylindrical housing 6 which is sealed at the top by a transparent frontmember 7 and at the bottom is similarly sealed in water tight fashion bya threadably connected floor or bottom 8 equipped with sealing O-rings25. The cylindrical housing 6 is preferably formed of a synthetic ma.-terial or plastic. Within the housing 6 there is arranged in uprightposition a tubular shaped storage vessel 2 or the like. This storagevessel 2 contains an activation solution 3. The electrode measuring ormeasurement chain 1 is constituted for instance, by a singlerodmeasuring chain with a glass electrode and a reference electrode wirein the same tube. Mounted at the lower end of the storage vessel 2 is asilicone rubber profile member 24. Arranged centrally of this profilemember 24 is a microswitch S which can be actuated by exerting pressureupon the diaphragm or membrane 14. As illustrated for instance,microswitch S can consist of a cylindrical metal portion serving as astationary switching element ll7,in which there is displaceably mountedfor up and down movement a movable switch member 16. By means of apressure spring 18 the movable switch member 16 is pressed with itsupper end against the rubber diaphgram of member 24 which pressesagainst membrane 14 of the storage vessel 2. 1

The microswitch S is closed if the measuring chain 1 in the storagevessel 2 is pulled upwardly at the edge of the tube and the measuringchain 1 bears with its laterally extending glass tube 4 for instanceupon such edge. By displacing. the measuring chain 1 with its glass tube4 into the storage vessel 2 within the slot 5 the switch isautomatically opened. A carrier or support plate is arranged within thehousing 6 beneath the housing front 7. This support plate 10 may bethreadahly connected for instance, and while using spacer elements 11,from the under side of the housing front 7 with a carrier handle 12located at such housing front 7, as shown. At the carrier plate 10 thereis mounted as the indicator device, for instance a moving coil measuringsystem M, and further an indicator cartridge containing self-indicatingsilica gel or some other. suitable material for maintaining the interiorof the housing dry'.

In the illustrated embodiment the amplifier circuit arrangement 13consists of a printed circuit board or plate 22 equipped with integratedcircuit components and is secured to the floor of the housing of therotary coil measuring system M. Further this amplifier circuitarrangement 13 supports two trimmer resistors R, and R, which form thefixedly balanced or matched resisresistance. Furthermore, housing 6 hasan annular groove 9 at which there can be wound up the measuring cable211 when the device is not used.

The temperature-dependency of a pH-value in all measurement media issuch that in. practice it is recommended to basically only perform C ina given temperature range. To this end, the measuring device isadvantageously adjusted such that the greatest measurement accuracyexists at the average temperature valued the range. Since mostmeasurements are performed at room temperature the measuring device orapparatus is preferably designed such that it provides the maximummeasuring accuracy at 25C. (or 37C) and it is prescribed thatduringmeasurement of the temperature of a material that its temperature shouldlie in a range of about 20C to 30C (or about 30C to 45C). If there isused a measurement amplifier which functions stabily and does notexhibit any temperature drift.

when working in the prescribed temperature range, for instance withtemperature fluctuations of 25C i 15C, then, with fixedly balancedresistors R R the used measuring chain, apart from exhibiting a definedpoint of intersection of the isotherms, must also possess a stable chainnull point, a constant asymmetrical potential and a constant slope, inorder to be able to obtain satisfactorily accurate measurement results.

According to the invention measuring chains or mea surement chainspossessing the previously mentioned properties and commerciallyavailable on the market are maintained in the storage vessel 2 under theaction of an activation solution 3, so that in each case an electrolytepossessing the special properties is located in the reference electrodecomponent or portion of the measuring chain and externally thereof. Theactivation solution 3 and the conductive electrolyte in the referenceelectrode component have the same chemical composition and contain asthe stabilization additive, according to the invention, at least two ofthe already discussed substances. By virtue of the stabilizationadditives the potential stability (asymmetrical potential) and thelongevity of the measuring chain is prolonged and the diaphragm of thereference electrode component of the measuring chain remains intact.Chemical depositions upon the surface of the glass membrane and at thediaphragm of the measuring chain are pre vented, there can be used forflushing tap water, and the stabilization additives also retard fungusgrowth during longer periods of storage in the vessel 2.

For phi-measurements there is preferably used a glasselectrode-measuring chain possessing an internal buffer of higher buffercapacity, a clearly defined intersection point of the isotherms at theneighborhood of or at a pH 7.0 and a constant electrical chain nullpoint lE(T) 0 (mV as well as stable slope or gradient of the measuringelectrode. A particularly suitable glass tors'of the amplifier circuitarrangement, as will be de- I electrode-measuring chain is an impactresistant single rod-measuring chain possessing these properties orcharacteristics and activation according to the invention. t I

When working with a single rod-measuring chain and when storingthemeasuring chain inthe storage vessel 2 the glass membrane is alsoimmersed in the activation I solution, so that also such is maintainedcontinuously under the same conditions. The potassium chlorideconcentration of the activation solution is adjusted such that owing toits ion activity there results at the measuring chain a chain null pointwhich is stable throughout a wide temperature range.

The function of the reference electrodehalf cell (electrodes of the 2ndtype), for instance the silver/silver chloride/KCl, occurs in accordancewith the solubility product L a w a wherein, L solubility product, and aion activity.

The chloride ion activity is maintained constant by the activationsolution with KCl:

a k, wherein a L/k is constant. According to the Nernst equation thepotential E equals:

E=E n T/"Fln L/ac1-, wherein,

R gas constant F Faradays constant T temperature n ion valence, andcalculated for 25C results in the equation:

+ 0.05916 log L/k,

that is, the potential of the reference electrode is constant owing tothe high chloride ion concentration in the activation solution anduninfluenced by the additives.

During the series manufacture of certain single rodmeasuring chains theasymmetrical potential of the individual chains scatters or varies by amaximum of i 0.05 pH-units or by i 3 mV from the electrical chain nullpoint and this stray value can be compensated and allowed to remainwithout further consideration during the first calibration of theapparatus by adjustingthe corresponding fixedly balanced resistor R,.The potential changes of the half cells in the measuring chain, such asfor instance the half cell Ag/AgCl/KCl or Hg/Hg,Cl,/KCl, only amount to:t 3 mV during temperature fluctuations of for instance C to 30C. Whenusing measuring chains with identical half cells in the glassandreference electrode components such deviations are completelycompensated.

in order to be able to measure with the utmost accuracy at C, with thepreviously described apparatus there is adjusted by means of the trimmerresistors R and R, a fixed electrical null point in the neighborhood ofor at pH 7.0 and a slope of 59.1 mV/pH-unit, which from the Nernstequation is equal to the theoretical slope of the glass electrode for25C calculated therefrom. With a measuring chain, the isothermicintersection point of which is clearly defined and which possesses aslope of 59.1 mV/pH-unit owing to the inventive activation, maximummeasurement uncertainties arise for different temperatures ofthe.measurement substance, which have been tabulated in the following table:

The above table is also valid if there is chosen instead of the fixedconstruction for 25C, 37C for instance. As will be apparent from thetable, notwithstanding fixed balanced measuring amplifiers there resultin practice insignificant deviations of the measurement values from themost exact measurement values at 25C. The alkali error is negligiblysmall within the employed pH-scale and the modern measuring chains.Mathematically the measurement uncertainty at 25C amounts to 0.00pH; thevalues given take into account the diffusion potentials at the diaphragmof the reference electrode and other fluctuations of small value whichoccur in practice. A reduction in the slope can be again eliminated bycleaning the active portion of the measuring chain and the inventiveactivation. As a practical matter the slope reduction due to changes inthe membrane glass only occurs after years.

Turning now to FIG. 2 there is illustrated therein the circuit diagramof an amplifier which has proven itself particularly useful in thepractical construction of the previously described measuring apparatus.The basic circuitry of the amplifier is known: a symmetricaldifferential amplifier possessing a respective field-effect transistorFET l and FET 2 as the input amplifier element. At the field-effecttransistors FET l and PET 2 there are connected in cascade thetransistors T3 and T4 of a further amplifier stage. The emitters ofthese transistors T3 and T4 form the outputs Al and A2 of thedifferential amplifier. With the circuitry depicted in FIG. 2 theindicator device M indicates the measurement result in digital form andaccordingly connected ahead of the indicator device M is an analoguedigital converter A/D serving as signal processing element 1. Ascontemplated by the invention the output A1 of the differentialamplifier is connected with the output element I through the agency of afixedly adjusted, variable resistor R2.

The measuring electrode wire of the measuring chain is directlyconnected via the input E2 with the gate electrode G2 of thefield-effect transistor FET 2, whereas the reference electrode wire ofthe measuring chain is connected via the input El and a resistor networkwith the gate electrode G1 of the other fieldeffect transistor FET 1.This resistor network or resistance network will be'seen to contain afixedly balanced or matched voltage divider containing the partialresistors R, and R, and R and R wherein the partial resistors R,. and R,are advantageously formed by a trimmer resistor R in order to be able toeasily undertake the initial calibration of the apparatus. The trimmerresistor R is bridged by two series connected resistors R, and R thejunction point of which is connected with the input E. The slide or tapof the trimmer resistor R, is coupled by means of a resistor R, with thegate electrode G1 of the field-effect transistor FET 1. Additionally,the gate electrodes G1 and G2 of both field-effect transistors areconnected without or by means of a respective capacitor C and C, withthe supply conductor connected to the positive pole or terminal of theoperating voltage source.

The resistor R, together with the capacitor C, forms an RC-element andthe capacitor C, together with the internal resistance of the measuringelectrode of the measuring chain forms a further RC-element. Both ofthese RC-elements serve to dampen the switch-in pulses and additionallyby virtue of such together with the field-effect transistors FET 1 andPET 2 and the transistors T3 and T4 the difi'erential amplifier isrendered symmetrical. The switch S arranged at the storage vessel 2 islocated at the supply conductor of the amplifier carrying positivepotential. The operating voltage is stabilized by a Zener diode Z and aresistor R The reference electrode input E insulated from ground andchassis is connected in alternating current fashion to ground by meansof capacitor C in order to afford the possibility to be able to carryout measuring operations both for grounded as well as ungrounded samplesubstances, during operation of the electric circuitry with networkpower.

As explained above, the described apparatus is particularly used formeasurement of pH-values. If there are connected ion selective measuringchains then it is possible to measure also other ion activities, such asp(Me)-values, electrode potentials (Redox potentials), and so forth. Theindividual apparatuses are only designed in each case for a singlemeasuring magnitude. To carry out the measuring operation it is onlynecessary to remove the measuring chain from the storage vessel with theactiviation or activation solution, whereupon the amplifier isimmediately automatically turned-on, and as has been shown from a greatnumber of trials exact and reproducible measurement values areindicated.

While there is shown and described present preferred embodiments of theinvention, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the scope of the following claims. ACCORD- INGLY,

What is claimed is:

1. An improved apparatus for rapid electrometric determination of ionicactivities, especially the pHvalue, p(Me)-value, or p(L)-value ofsubstancesin water containing systems by means of a measuring circuit,comprising an ion sensitive sensor which is an electrode-cellassemblyincluding a. a measuring electrode filled with a buffer solution havingan ion sensitive member, a measuring electrode wire in said electrodefilling and b. a reference electrode with a reference electrode wirewhich is immersed in an aqueous solution containing ions which determinethe standard potential of said reference electrode;

an amplifier circuit arrangement including an indicator device connectedto said electrodes;

said aqueous solution containing substances which provide a buffercapacity of said aqueous solution in addition to said ion determiningthe standard potential;

a storage means to hold said sensor when not in use;

an activation solution in said storage means within which said sensor isimmersed when held therein; said amplifier circuit arrangement includingpreset trimming resistor means to compensate for the asymmetry potentialand the temperature dependency of said sensor and to determine theamplification factor of said amplifier circuit arrangement.

2. Apparatus as defined in claim 1, characterized in that the bufferedaqueous solution containing the ions determining the standard potentialof said reference electrode as well as said activation solution of saidstorage vessel contain a stabilization additive.

3. Apparatus as defined in claim 2, characterized in that saidactivation solution is said storage means and said buffered aqueoussolution in which said reference electrode wire is immersed have thesame chemical composition.

5. Apparatus as defined in claim 4, characterized in that the ionsdetermining the standard potential of said reference electrode arechloride and that said reference electrode wire is a silver/silverchloride electrode wire.

6. Apparatus as defined in claim 5, characterized in that said aqueoussolution into which the reference electrode is immersed contains silverchloride until saturation and also activation solution.

7. Apparatus as defined in claim 6, characterized in that said bufferedaqueous solution as well as said activation solution containadditionally about 22 percent potassium chloride, about 2.0 percentmonochloro acetic acid and about 2.8 percent sodium chloro acetate.

8. Apparatus as defined in claim 6, characterized in that said aqueoussolution as well as said activation solution additionally contain about23 percent potassium chloride, about 0.6 percent acetic acid, about 0.8percent sodium acetate.

9. Apparatus as defined in claim 6, characterized in that said aqueoussolution as wellas said activation solution contain about 23.5 percentpotassium chloride, about 0.34 percent potassium dihydrogen phosphate,about 0.36 percent disodium hydrogen phosphate an about 0.1 percentthymol.

10. Apparatus as defined in claim 1, characterized in that saidmeasuring electrode is one having extremely high internal resistance.

1 1. Apparatus as defined in claim 1, characterized in that saidmeasuring electrode is one having low internal resistance.

12. Apparatus as defined in claim 1, wherein-said amplifier circuitarrangement comprises a bridge circuit incorporating two amplifiers,each. amplifier being provided with a respective field-effecttransmitterserving as an input amplifying element and an output, an output elementelectrically connected with said outputs of said amplifiers and servingfor the analogue or digital processing of the output signals, theindicator device being coupled with said output element, and wherein thegate electrode of the field-effecttransistor of one of said amplifiersof said bridge circuit is electrically coupled with a preset voltagedivider means for compensating the chain asymmetry potential and itsamplifier output is connected with said output element through theagency of a preset variable resistor for compensating the temperaturedependency of the sensor and the amplification factor, said voltagedivider means and said variable resistor containing said preset resistormeans.

13. Apparatus as defined in claim 12, further including a capacitor forgrounding the signal input of the ampliiicr of the bridge circuitcontaining said preset resistor means in order to be able to carry outmeasure ments with grounded and ungrounded sample substances, duringgrounding of the apparatus housing.

14. Apparatus as defined in claim 12, further including an RC-element,and wherein the gate electrode of the field-effect transistor of saidone amplifier of the bridge circuit is connected with said presetvoltage divider by means of the resistor of said RC-element, and acapacitor connected with the gate electrode of the field-effecttransistor of the other amplifier, said lastmentioned capacitor togetherwith the resistance of the electrode of said measuring chain connectedto said gate electrode of said other amplifier forming a secondRC-element.

15. Apparatus as defined in claim 14, further including a closed housingwithin which there is housed said amplifier circuit arrangement togetherwith said output element, said storage vessel for the activationsolution and the sensor being arranged in said housing, said storagevessel being provided with a switch mechanism in order to switch-on saidamplifier circuit arrangement upon removal of the sensor from saidstorage vessel and to switch-off said amplifier circuit arrangement uponinsertion of said sensor into said storage vessel.

16. Apparatus as defined in claim 15, wherein said storage vesselincludes a floor provided with a silicone rubber profile memberpossessing diaphragm means sealing in water-tight fashion said hollowcompartment, and a switch arranged in said hollow compartment, saidswitch being actuatable when pressure is exerted upon said disphragmmeans.

i t I i i

1. An improved apparatus for rapid electrometric determination of ionicactivities, especially the pH-value, p(Me)-value, or p(L)-value ofsubstances in water containing systems by means of a measuring circuit,comprising an ion sensitive sensor which is an electrode-cell-assemblyincluding a. a measuring electrode filled with a buffer solution havingan ion sensitive member, a measuring electrode wire in said electrodefilling and b. a reference electrode with a reference electrode wirewhich is immersed in an aqueous solution containing ions which determineThe standard potential of said reference electrode; an amplifier circuitarrangement including an indicator device connected to said electrodes;said aqueous solution containing substances which provide a buffercapacity of said aqueous solution in addition to said ion determiningthe standard potential; a storage means to hold said sensor when not inuse; an activation solution in said storage means within which saidsensor is immersed when held therein; said amplifier circuit arrangementincluding preset trimming resistor means to compensate for the asymmetrypotential and the temperature dependency of said sensor and to determinethe amplification factor of said amplifier circuit arrangement. 2.Apparatus as defined in claim 1, characterized in that the bufferedaqueous solution containing the ions determining the standard potentialof said reference electrode as well as said activation solution of saidstorage vessel contain a stabilization additive.
 3. Apparatus as definedin claim 2, characterized in that said activation solution is saidstorage means and said buffered aqueous solution in which said referenceelectrode wire is immersed have the same chemical composition. 4.Apparatus as defined in claim 3, characterized in that the stabilizationadditives of the aqueous solution that is said activation solutioncomprises at least two of the following substances: alkali hydrogenphthalates, alkali tetraborates, alkali phosphates, alkali hydrogenphosphates, thymol, alkylesters of hydroxy benzoic acids, alkalibenzoates, citric acid, acetic acid, monochloro acetic acid, alkalisalts of acetic and monochloro acetic acid.
 5. Apparatus as defined inclaim 4, characterized in that the ions determining the standardpotential of said reference electrode are chloride and that saidreference electrode wire is a silver/silver chloride electrode wire. 6.Apparatus as defined in claim 5, characterized in that said aqueoussolution into which the reference electrode is immersed contains silverchloride until saturation and also activation solution.
 7. Apparatus asdefined in claim 6, characterized in that said buffered aqueous solutionas well as said activation solution contain additionally about 22percent potassium chloride, about 2.0 percent monochloro acetic acid andabout 2.8 percent sodium chloro acetate.
 8. Apparatus as defined inclaim 6, characterized in that said aqueous solution as well as saidactivation solution additionally contain about 23 percent potassiumchloride, about 0.6 percent acetic acid, about 0.8 percent sodiumacetate.
 9. Apparatus as defined in claim 6, characterized in that saidaqueous solution as well as said activation solution contain about 23.5percent potassium chloride, about 0.34 percent potassium dihydrogenphosphate, about 0.36 percent disodium hydrogen phosphate and about 0.1percent thymol.
 10. Apparatus as defined in claim 1, characterized inthat said measuring electrode is one having extremely high internalresistance.
 11. Apparatus as defined in claim 1, characterized in thatsaid measuring electrode is one having low internal resistance. 12.Apparatus as defined in claim 1, wherein said amplifier circuitarrangement comprises a bridge circuit incorporating two amplifiers,each amplifier being provided with a respective field-effect transmitterserving as an input amplifying element and an output, an output elementelectrically connected with said outputs of said amplifiers and servingfor the analogue or digital processing of the output signals, theindicator device being coupled with said output element, and wherein thegate electrode of the field-effect transistor of one of said amplifiersof said bridge circuit is electrically coupled with a preset voltagedivider means for compensating the chain asymmetry potential and itsamplifier output is connected with said output element through theagency of a presEt variable resistor for compensating the temperaturedependency of the sensor and the amplification factor, said voltagedivider means and said variable resistor containing said preset resistormeans.
 13. Apparatus as defined in claim 12, further including acapacitor for grounding the signal input of the amplifier of the bridgecircuit containing said preset resistor means in order to be able tocarry out measurements with grounded and ungrounded sample substances,during grounding of the apparatus housing.
 14. Apparatus as defined inclaim 12, further including an RC-element, and wherein the gateelectrode of the field-effect transistor of said one amplifier of thebridge circuit is connected with said preset voltage divider by means ofthe resistor of said RC-element, and a capacitor connected with the gateelectrode of the field-effect transistor of the other amplifier, saidlast-mentioned capacitor together with the resistance of the electrodeof said measuring chain connected to said gate electrode of said otheramplifier forming a second RC-element.
 15. Apparatus as defined in claim14, further including a closed housing within which there is housed saidamplifier circuit arrangement together with said output element, saidstorage vessel for the activation solution and the sensor being arrangedin said housing, said storage vessel being provided with a switchmechanism in order to switch-on said amplifier circuit arrangement uponremoval of the sensor from said storage vessel and to switch-off saidamplifier circuit arrangement upon insertion of said sensor into saidstorage vessel.
 16. Apparatus as defined in claim 15, wherein saidstorage vessel includes a floor provided with a silicone rubber profilemember possessing diaphragm means sealing in water-tight fashion saidhollow compartment, and a switch arranged in said hollow compartment,said switch being actuatable when pressure is exerted upon saiddisphragm means.